Diverter confusion in Ohio

Sorry for the duplicate post from the "General" forum, but missed this forum where my question really should be:

Hello to all,

First off, my apologies if this becomes long and a bit disjointed. Confusion reigns supreme in my plumbing knowledge.

I'm having a problem with the diverter stem on my shower. I believe it is a Briggs unit because around two years ago, I replaced both the hot and cold stems with Briggs stems (534NS) from Dinapolli Plumbing online. I also orders the diverter stem, which I'm assuming was also a Briggs. Just recently, I noticed that when the diverter valve was turned counter-clockwise to 'activate' the shower, cold water started leaking out of the cold water stem (through the escutheon). I removed the diverter and noticed that a washer (not sure what this one is called) over the stem that sits against the bonnet nut was dislodged and just running loose over the stem. Could this be causing the leak? What is this washer called and where might I find it. I could not locate this washer anywhere at Home Depot or Lowes. And if all else fails, does anyone know where I might find a new diverter?

Lastly, does anyone have a good diagram of a diverter and the pieces/washers attached?